The present invention relates to voltage measurement and, in particular, to circuitry and methods for obtaining accurate voltage measurements in battery powered electronic devices such as, for example, medical implantible devices.
The need to measure battery voltage accurately in medical implantible devices will be quickly acknowledged. It is critical that such devices provide for periodic and accurate measurement not only of battery voltage but also of a number of other voltages which may range from ground upwards. Examples of voltage levels which require periodic checking include residual capacitor voltage on completion of a pace pulse in pacemaker technology or of a high voltage pulse in defibrillator technology. Another common requirement is that small differences in voltage be measurable with an overall accuracy which is greater than the absolute accuracy of individual voltage measurements. Hence, monotonicity and repeatability are critical features of any measurement circuit utilized in these applications. A final implicit requirement is that very low power operation be possible, albeit at very slow measurement speeds.
The analog to digital conversion voltage measurement technique which is the subject of the present invention is a modified version of the well-known so-called "single slope" method. This type of A/D converter is inherently monotonic. Repeatability is limited primarily by comparator noise.
The "single-slope" technique operates as follows:
First, a known reference voltage V.sub.ref is sampled and held onto a capacitor. Next, a highly linear current source is connected across the capacitor and the timne taken for the capacitor to discharge to a known level V.sub.0 (typically ground) is measured (T1). Then the unknown voltage Vx is sampled and held onto the same capacitor. The linear current source is then connected across the capacitor and, again, the time taken to discharge to V.sub.0 is measured (T2). Finally, the unknown voltage Vx is calculated as follows: EQU V.sub.X =T2/T1 (V.sub.ref -V.sub.0)+V.sub.0 ( 1)
In the case where V.sub.0= 0, then EQU V.sub.x =T2/T1 (v.sub.ref).
In general, the use of ground as V.sub.0 requires the use of an auxiliary negative power supply. This option is obviously not available in battery powered devices. The alternative is to utilize a positive V.sub.0. This can be difficult since not only must this V.sub.0 be fixed, but also, as can be deduced from Equation (1) above, it must be known to a high degree of precision. Hence, it effectively must be a second reference voltage. Use of this second known voltage level rules out measurement of voltages below that level.